Many medical tests, including Papanicolaou (Pap) smears, require a physician to collect cells by brushing and/or scraping a skin or mucous membrane in a target area with an instrument. The cells are then smeared onto a slide, and are fixed and transported to a laboratory where the slide is stained. The slide can then be examined under a microscope by a cytotechnologist and/or a pathologist to identify cellular abnormalities. During evaluation, a pathologist may employ a polychrome technique, characterized by staining the nuclear part of the cells, to determine the presence of dysplasia or neoplasia. The pathologist may also apply a counter-stain for viewing the cytoplasm of the cells. Because the sample may contain debris, blood, mucus, and other obscuring artifacts, the test may be difficult to evaluate, and may not provide an accurate diagnostic assessment of the collected sample.
Cytology based on the collection of the exfoliated cells into a liquid preservative offers many advantages over the traditional method of smearing the cells directly onto the slide. A slide can be prepared from the cell suspension using a filter transfer technique, such as the Cytospin® technique and the Thin-prep® technique, as disclosed in U.S. Pat. Nos. 6,634,244, 6,572,824, 6,562,299, 6,318,190, 6,225,125, 6,010,909, 5,942,700, 5,772,818, 5,503,802, 5,364,597, and 5,143,627, which are expressly incorporated herein by reference for all that they teach and disclose.
Filter transfer methods generally start with a collection of cells suspended in a liquid. These cells may be collected and dispersed into a liquid preservative or they may naturally exist in a collected biological liquid. Dispersion in liquid preservatives containing methanol, such as PreservCyt™ solution, breaks up mucus and lyses red blood cells and inflammatory cells, without affecting the cells of interest. The liquid is then passed through a filter with an aperture covered by a membrane to concentrate and collect the cells. Debris, such as lysed blood cells and dispersed mucus, which flow through the pores of the membrane, are not collected on the membrane and are greatly reduced by the combined method of dispersion and filtering. Then, the cells collected on the membrane are transferred onto a slide. Existing filter transfer methods typically transfer cells from the membrane to the slide in a “semi-dry” environment, i.e., in an environment where a majority of the fluid has been removed from the filter assembly.